positionable: add containment

I think there should be a way to pass a container element that will limit position with its borders (besides window). the Option may be called containment.

And I actually don't understand logic behind code:

positinable.js (367):
within = $(isEvent || !options.of ? window : options.of),

in what case does within limitation works?

Start with positionable.js around line 367 and trace how options.of determines the current positioning boundary. Clarify how a containment option should accept a container element and define the expected edge behavior, then verify that positioning is limited by the container rather than only the window.

Written by the indexing model from the issue text.